Diffusion barrier coatings having graded compositions and devices incorporating the same

ABSTRACT

A composite article comprises a substrate having at least a substrate surface and a graded-composition coating disposed on a substrate surface. The composition of the coating material varies substantially continuously across its thickness. The coating reduces the transmission rates of oxygen, water vapor, and other chemical species through the substrate such that the composite article can be used effectively as a diffusion barrier to protect chemically sensitive devices or materials. An organic light-emitting device incorporates such a composite article to provide an extended life thereto.

BACKGROUND OF INVENTION

The present invention relates generally to composite films havingimproved resistance to diffusion of chemical species and to devicesincorporating such composite films. In particular, the present inventionrelates to light-emitting devices having at least an organicelectroluminescent material that incorporate such composite films andhave improved stability in the environment.

Electroluminescent (“EL”) devices, which may be classified as eitherorganic or inorganic, are well known in graphic display and imaging art.EL devices have been produced in different shapes for many applications.Inorganic EL devices, however, typically suffer from a required highactivation voltage and low brightness. On the other hand, organic ELdevices (“OELDs”), which have been developed more recently, offer thebenefits of lower activation voltage and higher brightness in additionto simple manufacture, and, thus, the promise of more widespreadapplications.

An OELD is typically a thin film structure formed on a substrate such asglass or transparent plastic. A light-emitting layer of an organic ELmaterial and optional adjacent semiconductor layers are sandwichedbetween a cathode and an anode. The semiconductor layers may be eitherhole (positive charge)-injecting or electron (negative charge)-injectinglayers and also comprise organic materials. The material for thelight-emitting layer may be selected from many organic EL materials. Thelight emitting organic layer may itself consist of multiple sublayers,each comprising a different organic EL material. State-of-the-artorganic EL materials can emit electromagnetic (“EM”) radiation havingnarrow ranges of wavelengths in the visible spectrum. Unlessspecifically stated, the terms “EM radiation” and “light” are usedinterchangeably in this disclosure to mean generally radiation havingwavelengths in the range from ultraviolet (“UV”) to mid-infrared(“mid-IR”) or, in other words, wavelengths in the range from about 300nm to about 10 micrometer. To achieve white light, prior-art devicesincorporate closely arranged OELDs emitting blue, green, and red light.These colors are mixed to produce white light.

Conventional OELDs are built on glass substrates because of acombination of transparency and low permeability of glass to oxygen andwater vapor. A high permeability of these and other reactive species canlead to corrosion or other degradation of the devices. However, glasssubstrates are not suitable for certain applications in whichflexibility is desired. In addition, manufacturing processes involvinglarge glass substrates are inherently slow and, therefore, result inhigh manufacturing cost. Flexible plastic substrates have been used tobuild OLEDs. However, these substrates are not impervious to oxygen andwater vapor, and, thus, are not suitable per se for the manufacture oflong-lasting OELDs. In order to improve the resistance of thesesubstrates to oxygen and water vapor, alternating layers of polymericand ceramic materials have been applied to a surface of a substrate. Ithas been suggested that in such multilayer barriers, a polymeric layeracts to mask any defects in an adjacent ceramic layer to reduce thediffusion rates of oxygen and/or water vapor through the channels madepossible by the defects in the ceramic layer. However, an interfacebetween a polymeric layer and a ceramic layer is generally weak due tothe incompatibility of the adjacent materials, and the layers, thus, areprone to be delaminated.

Therefore, there is a continued need to have robust films that havereduced diffusion rates of environmentally reactive materials. It isalso very desirable to provide such films to produce flexible OELDs thatare robust against degradation due to environmental elements.

SUMMARY OF INVENTION

The present invention provides a substrate having at least a coatingdisposed on a surface thereof, which coating is capable of reducingdiffusion rates of chemical species therethrough. The coating comprisesa material the composition of which varies across a thickness thereof.Such a coating will be termed interchangeably hereinafter a“diffusion-barrier coating having graded composition,”“graded-composition diffusion-barrier coating,” “graded-compositionbarrier coating,” “diffusion-barrier coating,” or simply“graded-composition coating”.

In one aspect of the present invention, the substrate comprises apolymeric material.

In another aspect of the present invention, a region between thesubstrate and the coating is diffuse such that there is a gradual changefrom the composition of the bulk substrate to the composition of theportion of the coating adjacent to the substrate. In this embodiment, amaterial of the coating adjacent to the substrate penetrates into thesubstrate.

In still another aspect of the present invention, at least a substratehaving a diffusion-barrier coating having graded composition is includedin an assembly comprising a device sensitive to chemical species toprotect such an assembly from attack by these chemical species.

In still another aspect of the present invention, such a device is anOELD, which comprises a pair of electrodes and an organic light-emittinglayer sandwiched therebetween.

In yet another aspect of the present invention, an OELD is sandwichedbetween two films, each having a diffusion-barrier coating having gradedcomposition.

The present invention also provides a method for making a substratecoated with a diffusion barrier coating having a graded composition. Themethod comprises the steps of: (a) providing a substrate having asubstrate surface; (b) depositing a coating material having a firstcomposition on the substrate surface; and (c) changing a composition ofthe coating material substantially continuously such that thecomposition of the coating varies from the first composition to a secondcomposition across a thickness of the coating.

In another aspect of the present invention, a method for making anassembly comprising a device that is sensitive to chemical speciescomprises the steps of: (a) providing at least a substrate coated with adiffusion barrier coating having a graded composition; and (b) disposingthe device on the substrate.

In another aspect of the present invention, such a device is an OELD,and the method comprises the steps of: (a) providing at least asubstrate coated with a diffusion barrier coating having a gradedcomposition; (b) forming a first electrode on the substrate; (c) formingan organic light-emitting layer on the first electrode; and (d) forminga second electrode on the organic light-emitting layer.

In still another aspect of the present invention, an OLED comprising apair of electrodes and an organic light-emitting layer disposed betweenthe pair of electrodes and a substrate, coated with a diffusion barriercoating having a graded composition are laminated together to form alight source.

Other features and advantages of the present invention will be apparentfrom a perusal of the following detailed description of the inventionand the accompanying drawings in which the same numerals refer to likeelements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a deposition apparatus using theexpanding thermal-plasma chemical-vapor deposition.

FIG. 2 is a schematic diagram of the apparatus of FIG. 1 used in acontinuous deposition.

FIG. 3 is a schematic diagram of a deposition apparatus using theradio-frequency plasma-enhanced chemical-vapor deposition.

FIG. 4 shows the elemental composition at various depths of agraded-composition barrier coating of the present invention.

FIG. 5 compares the oxygen transmission rates through an uncoatedsubstrate and one that is coated with a graded-composition barriercoating.

FIG. 6 compares the water transmission rates through an uncoatedsubstrate and one that is coated with a graded-composition barriercoating.

FIG. 7 shows the relative light transmission through a substrate havinga graded-composition barrier coating compared to that through anuncoated substrate.

FIG. 8 shows schematically a device used with a substrate having agraded-composition barrier coating.

FIG. 9 shows schematically a construction of an OELD.

FIG. 10 shows another embodiment of an OELD including a hole injectionenhancement layer.

FIG. 11 shows another embodiment of an OELD including a hole injectionenhancement layer and a hole transport layer.

FIG. 12 shows another embodiment of an OELD including an electroninjecting and transporting layer.

FIG. 13 shows an OELD sealed between a substrate having agraded-composition barrier coating and a reflective layer.

FIG. 14 shows an OELD sealed between two substrates, each having agraded-composition barrier coating.

FIG. 15 shows a sealed OELD having a light conversion layer.

DETAILED DESCRIPTION

The present invention, in one aspect, provides a substrate having atleast a coating disposed on a surface thereof, which coating is capableof reducing diffusion rates of chemical species through the substrate.The coating comprises a material, the composition of which varies acrossa thickness thereof. Such a coated substrate finds uses in providingprotection to many devices or components; e.g., electronic devices, thatare susceptible to reactive chemical species normally encountered in theenvironment. In another example, such a substrate or film having adiffusion-barrier coating having graded composition can advantageouslybe used in packaging of materials, such as foodstuff, that are easilyspoiled by chemical or biological agents normally existing in theenvironment.

Organic light-emitting material and/or cathode materials in OELDs aresusceptible to attack by reactive species existing in the environment,such as oxygen, water vapor, hydrogen sulfide, SO_(x), NO_(x), solvents,etc. Films having a graded-composition diffusion-barrier coating areparticularly useful to extend the life of these devices and render themmore commercially viable. A barrier coating of the present invention maybe made by depositing reaction or recombination products of reactingspecies onto a substrate or film. Varying the relative supply rates orchanging the identities of the reacting species results in a coatingthat has a graded composition across its thickness. Thus, a coating ofthe present invention does not have distinct interfaces at which thecomposition of the coating changes abruptly. Such abrupt changes incomposition tend to introduce weak spots in the coating structure wheredelamination can easily occurs. Substrate materials that benefit fromhaving a graded-composition diffusion-barrier coating are organicpolymeric materials; such as polyethyleneterephthalate (“PET”);polyacrylates; polycarbonate; silicone; epoxy resins;silicone-functionalized epoxy resins; polyester such as Mylar (made byE.I. du Pont de Nemours & Co.); polyimide such as Kapton H or Kapton E(made by du Pont), Apical AV (made by Kanegafugi Chemical IndustryCompany), Upilex (made by UBE Industries, Ltd.); polyethersulfones(“PES,” made by Sumitomo); polyetherimide such as Ultem (made by GeneralElectric Company); and polyethylenenaphthalene (“PEN”).

Suitable coating compositions of regions across the thickness areorganic, inorganic, or ceramic materials. These materials are typicallyreaction or recombination products of reacting plasma species and aredeposited onto the substrate surface. Organic coating materialstypically comprise carbon, hydrogen, oxygen, and optionally other minorelements, such as sulfur, nitrogen, silicon, etc., depending on thetypes of reactants. Suitable reactants that result in organiccompositions in the coating are straight or branched alkanes, alkenes,alkynes, alcohols, aldehydes, ethers, alkylene oxides, aromatics, etc.,having up to 15 carbon atoms. Inorganic and ceramic coating materialstypically comprise oxide; nitride; carbide; boride; or combinationsthereof of elements of Groups IIA, IIIA, IVA, VA, VIA, VIIA, IB, andIIB; metals of Groups IIIB, IVB, and VB; and rare-earth metals. Forexample, silicon carbide can be deposited onto a substrate byrecombination of plasmas generated from silane (SiH₄) and an organicmaterial, such as methane or xylene. Silicon oxycarbide can be depositedfrom plasmas generated from silane, methane, and oxygen or silane andpropylene oxide. Silicon oxycarbide also can be deposited from plasmasgenerated from organosilicone precursors, such as tetraethoxysilane(TEOS), hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDSN), oroctamethylcyclotetrasiloxane (D4). Silicon nitride can be deposited fromplasmas generated from silane and ammonia. Aluminum oxycarbonitride canbe deposited from a plasma generated from a mixture of aluminum tartrateand ammonia. Other combinations of reactants may be chosen to obtain adesired coating composition. The choice of the particular reactants iswithin the skills of the artisans. A graded composition of the coatingis obtained by changing the compositions of the reactants fed into thereactor chamber during the deposition of reaction products to form thecoating.

Coating thickness is typically in the range from about 10 nm to about10000 nm, preferably from about 10 nm to about 1000 nm, and morepreferably from about 10 nm to about 200 nm. It may be desired to choosea coating thickness that does not impede the transmission of lightthrough the substrate, such as a reduction in light a transmission beingless than about 20 percent, preferably less than about 10 percent, andmore preferably less than about 5 percent. The coating may be formed byone of many deposition techniques, such as plasma-enhancedchemical-vapor deposition (“PECVD”), radio-frequency plasma-enhancedchemical-vapor deposition (“RFPECVD”), expanding thermal-plasmachemical-vapor deposition (“ETPCVD”), sputtering including reactivesputtering, electron-cyclotron-resonance plasma-enhanced chemical-vapordeposition (ECRPECVD”), inductively coupled plasma-enhancedchemical-vapor deposition (“ICPECVD”), or combinations thereof.

FIG. 1 schematically illustrates a reactor 10 and associated equipmentfor the ETPCVD technique. At least one cathode 20; typically made oftungsten, is disposed in a cathode housing 30. Anode plate 40 isdisposed at one end of cathode housing 30. Optionally, at least acathode housing is electrically floating. A voltage applied betweencathode 20 and anode 40 generates an arc for plasma generation. Acarrier gas, such as argon, is fed through line 50 to the arc. A plasmais generated and exits a nozzle or orifice 70 at the center of anode 40.A first reactant gas can be fed through line 60 into the carrier gasline at a point between cathode 20 and anode 40. A second reactant gasis fed through supply line 80 to a point downstream from orifice 70.Supply line 80 may also terminate with a perforated ring disposed withinexpanding plasma beam 84 for better mixing. Other reactant supply linescan be provided for different reactant species. Radicals are generatedfrom reactant gases, combined, carried to substrate 90, and depositedthereon, which substrate is supported on substrate holder 100. Substrateholder 100 is disposed opposite and at a distance from nozzle 70 and ismovable relative to nozzle 70 by substrate-holder shaft 110. Reactor 10is kept under vacuum via vacuum connection 112. For example, when thecoating on the substrate is desired to comprise silicon nitride, thefirst reactant gas can be ammonia, and the second reactant gas can besilane. The relative supply rates of first and second reactant gases arevaried during deposition to vary the composition of the depositedmaterial as the coating is built up. Although FIG. 1 schematically showsa substrate as a single piece 90, a coating may be deposited on acontinuous substrate in similar equipment. For example, FIG. 2 shows asupply roll 120 of a thin polymeric substrate 115, which supply roll 120is disposed on one side of substrate holder 100, and a take-up roll 122disposed on the other side of substrate holder 100. As roll 120continuously unwinds and roll 122 continuously winds, uncoated substratefilm 115 continuously receives the coating material as it passes oversubstrate holder 100. In another embodiment of the invention, substratefilm 115 passes through an area opposite to many overlapping plasmabeams, each being generated with different or varying compositions toreceive a coating, the composition of which varies continuously thoughits thickness.

In the ETPCVD technique, the plasma is generated at a high pressurecompared to the regular PECVD technique. The plasma in arc channel 65has a velocity on the order of sound velocity. The plasma expandssupersonically into reactor chamber 10 via nozzle 70 and movessubsonically toward substrate 90.

FIG. 3 schematically shows reactor 200 and associated equipment for theRFPECVD technique. Radio frequency (“RF”) power is applied to cathode210, which is disposed in reactor 200, by RF generator and amplifier 204and matching network 208, which comprises a plurality of electricaland/or electronic components for generating appropriate impedance orother electrical characteristics of the overall system to maximize powertransfer from RF generator and amplifier 204. Substrate 90 is disposedon substrate holder 100 opposite to cathode 210 to receive plasmadeposition. Substrate holder may be grounded or electrically coupled toanother RF generator and matching network, if a different potential isdesired. A reactant gas or a mixture of gases is fed into a gasdistributor 212 through a gas supply 214. Gas distributor 212 may haveany shape that promotes a substantially uniform distribution of gases.For example, it may be a ring having perforations directed towardsubstrate holder 100. Alternatively, cathode 210 may itself be hollowand porous and receives reactant gases. A plasma is generated andmaintained by the RF field and flows toward substrate 90. Precursorspecies in the plasma are combined and deposited on substrate 90. Thecomposition of the coating can be varied while it is built up by varyingthe composition of the reactant gas mixture fed into distributor 212. Acontinuous substrate such as a polymeric film may be coated with agraded-composition coating by providing an unwinding supply roll and atake-up roll, as described above. The substrate likewise can travelopposite to a plurality of deposition stations, which supply varying gascompositions, to produce a continuous film having a graded-compositioncoating.

ECRPECVD is another suitable deposition technique. This method operatesat low pressure, typically less than about 0.5 mm Hg, and typicallywithout electrodes. A discharge is generated by microwave. A magneticfield is used to create the resonance condition of the electron gas,which results in a very high degree of ionization due to electronacceleration at a distance away from the substrate. The low pressurepreserves a high number density of free radicals until the plasmareaches the substrate and prevents normally undesirable severebombardment thereof.

ICPECVD is another electrodeless deposition technique that can createhigh-density plasma at low pressure. A plasma is generated by anelectromagnetic field generated by a concentric induction coil disposedoutside one end of the deposition chamber. The substrate is disposed inthe deposition chamber at the opposite end. Deposition can typically becarried out at pressure much less than 0.5 mm Hg.

In another embodiment of the present invention, the energy of the ionsin a plasma may be controlled such that they penetrate into a surfacelayer of the substrate to create a diffuse transition region between thecomposition of the bulk substrate and the composition of the coating.Such a transition prevents an abrupt change in the composition andmitigates any chance for delamination of the coating.

A graded-composition coating having a thickness of about 500 nm wasformed on a polycarbonate substrate having a dimension of about 10 cm×10cm and a thickness of about 0.2 mm using the RFPECVD technique andtested for water vapor and oxygen transmission. Silane (maximum flowrate of about 500 standard cm³/minute), ammonia (maximum flow rate ofabout 60 standard cm³/minute), and propylene oxide (maximum flow rate ofabout 500 standard cm³/minute) were used to produce the graded coatingcomprising silicon, carbon, oxygen, and nitrogen. The rates of thereactant gases were varied during deposition so that the composition ofthe coating varied continuously across its thickness. The power fed tothe RF electrode was about 100 W when plasma was generated frompropylene oxide, and about 200 W when a mixture of silane and ammoniawas fed into the reactor. The vacuum level in the reactor was about 0.2mm Hg and the average temperature was about 55° C. FIG. 4 shows theelemental composition of the coating, as measured by dynamic XPS, as afunction of sputtering time to remove portions of the thickness of thecoating during the dynamic XPS testing, which is directly related to thedepth of the coating. Oxygen and water vapor transmission test resultsare shown in FIGS. 5 and 6. The oxygen transmission rate through thecoated plastic substrate was reduced by over three orders of magnitudecompared to the uncoated substrate, and the water vapor transmissionrate by over two orders of magnitude. Light transmission at variouswavelengths of the visible spectrum through the coated substrate isshown in FIG. 7. The reduction in light transmission in the blue to redregion (about 430 nm to about 700 nm) was generally less than 7 percent.

A plastic substrate coated with a graded-composition coating, which isformed by any method disclosed above can be advantageously used toproduce flexible light sources based on organic light-emittingmaterials. Other electronic devices that can benefit from the protectionafforded by a graded-composition coating are, for example, displaysincluding liquid crystal displays, photovoltaic devices, flexibleintegrated circuits, or components of medical diagnostic systems. Theterm “flexible” means being capable of being bent into a shape having aradius of curvature of less than about 100 cm. The term “substantiallytransparent” means allowing a total transmission of at least about 50percent, preferably at least about 80 percent, and more preferably atleast 90 percent, of light in the visible range (i.e., having wavelengthin the range from about 400 nm to about 700 nm). It should be understoodthat the composition of a graded-composition barrier coating does notnecessarily vary monotonically from one surface to the other surfacethereof. A monotonically varying composition is only one case ofgraded-composition for the barrier of the present invention.

FIG. 8 is a schematic diagram of an embodiment of the present invention.It should be understood that the figures accompanying this disclosureare not drawn to scale. OELD or a light-emitting device 310 comprises anorganic EL member 320 disposed on a substantially transparent substrate340 having a graded-composition barrier coating 350, as described above.The graded-composition barrier coating 350 may be disposed or otherwiseformed on either or both of the surfaces of the substrate 340 adjacentto the organic EL member 320. Preferably, the graded-composition barriercoating 350 is disposed or formed on the surface of the substrate 340adjacent to the organic EL member 320 or it may completely cover thesubstrate 340. Although FIG. 8 shows schematically a distinct interfacebetween substrate 340 and coating 350, such a coating may be formed suchthat there is no sharp interface therebetween, as described above.

Substrate 340 may be a single piece or a structure comprising aplurality of adjacent pieces of different materials and has an index ofrefraction (or refractive index) in the range from about 1.05 to about2.5, preferably from about 1.1 to about 1.6. Preferably, substrate 340is made of a substantially transparent polymeric material. Examples ofsuitable polymeric materials are polyethylenterephathalate (“PET”),polyacrylates, polycarbonate, silicone, epoxy resins,silicone-functionalized epoxy resins, polyester, polyimide,polyetherimide, PES, PEN, polynorbonenes, or poly (cyclic olefins).

Light-emitting member 320 comprises at least one layer 330 of at leastone organic EL material sandwiched between two electrodes 322 and 338,as shown in FIG. 9. As will be disclosed below, the light-emittingmember may comprise one or more additional layers between an electrodeand the layer 330 of organic EL material. When a voltage is supplied bya voltage source 326 and applied across electrodes 322 and 338, lightemits from the organic EL material. In a preferred embodiment, electrode322 is a cathode injecting negative charge carriers (electrons) intoorganic EL layer 330 and is made of a material having a low workfunction; e.g., less than about 4 eV. Low-work function materialssuitable for use as a cathode are K, Li, Na, Mg, La, Ce, Ca, Sr, Ba, Al,Ag, In, Sn, Zn, Zr, Sm, Eu, alloys thereof, or mixtures thereof.Preferred materials for the manufacture of cathode layer 322 are Ag—Mg,Al—Li, In—Mg, and Al—Ca alloys. Layered non-alloy structures are alsopossible, such as a thin layer of a metal such as Ca (thickness fromabout 1 to about 10 nm) or a non-metal such as LiF, covered by a thickerlayer of some other metal, such as aluminum or silver. In thisembodiment, electrode 338 is an anode injecting positive charge carriers(or holes) into organic layer 330 and is made of a material having ahigh work function; e.g., greater than about 4.5 eV, preferably fromabout 5 eV to about 5.5 eV. Indium tin oxide (“ITO”) is typically usedfor this purpose. ITO is substantially transparent to light transmissionand allows at least 80% light transmitted therethrough. Therefore, lightemitted from organic electroluminescent layer 330 can easily escapethrough the ITO anode layer without being seriously attenuated. Othermaterials suitable for use as the anode layer are tin oxide, indiumoxide, zinc oxide, indium zinc oxide, cadmium tin oxide, and mixturesthereof. In addition, materials used for the anode may be doped withaluminum or fluorine to improve charge injection property. Electrodelayers 322 and 338 may be deposited on the underlying element byphysical vapor deposition, chemical vapor deposition, ion beam-assisteddeposition, or sputtering. A thin, substantially transparent layer of ametal is also suitable.

Although the preferred order of the cathode and anode layers 322 and 338is disclosed above, the electrode layers may be reversed. Electrodelayers 322 and 338 may serve as the anode and cathode, respectively.Typically, the thickness of the cathode layer in this case is about 200nm.

Organic EL layer 330 serves as the transport medium for both holes andelectrons. In this layer these excited species combine and drop to alower energy level, concurrently emitting EM radiation in the visiblerange. Organic EL materials are chosen to electroluminesce in thedesired wavelength range. The thickness of the organic EL layer 330 ispreferably kept in the range of about 100 to about 300 nm. The organicEL material may be a polymer, a copolymer, a mixture of polymers, orlower molecular-weight organic molecules having unsaturated bonds. Suchmaterials possess a delocalized π-electron system, which gives thepolymer chains or organic molecules the ability to support positive andnegative charge carriers with high mobility. Suitable EL polymers arepoly(N-vinylcarbazole) (“PVK”, emitting violet-to-blue light in thewavelengths of about 380-500 nm); poly(alkylfluorene) such as poly(9,9-dihexylfluorene) (410-550 nm), poly(dioctylfluorene) (wavelength atpeak EL emission of 436 nm), orpoly{9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl} (400-550 nm);poly(praraphenylene) derivatives such as poly(2-decyloxy-1,4-phenylene)(400-550 nm). Mixtures of these polymers or copolymers based on one ormore of these polymers and others may be used to tune the color ofemitted light.

Another class of suitable EL polymers is the polysilanes. Polysilanesare linear silicon-backbone polymers substituted with a variety of alkyland/or aryl side groups. They are quasi one-dimensional materials withdelocalized σ-conjugated electrons along polymer backbone chains.Examples of polysilanes are poly(di-n-butylsilane),poly(di-n-pentylsilane), poly(di-n-hexylsilane),poly(methylphenylsilane), and poly{bis(p-butylphenyl)silane} which aredisclosed in H. Suzuki et al., “Near-Ultraviolet ElectroluminescenceFrom Polysilanes,” 331 Thin Solid Films 64-70 (1998). These polysilanesemit light having wavelengths in the range from about 320 nm to about420 nm.

Organic materials having molecular weight less than about 5000 that aremade of a large number of aromatic units are also applicable. An exampleof such materials is 1,3,5-tris{n-(4-diphenylaminophenyl)phenylamino}benzene, which emits light in the wavelength range of380-500 nm. The organic EL layer also may be prepared from lowermolecular weight organic molecules, such as phenylanthracene,tetraarylethene, coumarin, rubrene, tetraphenylbutadiene, anthracene,perylene, coronene, or their derivatives. These materials generally emitlight having maximum wavelength of about 520 nm. Still other suitablematerials are the low molecular-weight metal organic complexes such asaluminum-, gallium-, and indium-acetylacetonate, which emit light in thewavelength range of 415-457 nm, aluminum-(picolymethylketone)-bis{2,6-di(t-butyl)phenoxide} orscandium-(4-methoxy-picolylmethylketone)-bis(acetylacetonate), whichemits in the range of 420-433 nm. For white light application, thepreferred organic EL materials are those emit light in the blue-greenwavelengths.

More than one organic EL layer may be formed successively one on top ofanother, each layer comprising a different organic EL material thatemits in a different wavelength range. Such a construction canfacilitate a tuning of the color of the light emitted from the overalllight-emitting device 310.

Furthermore, one or more additional layers may be included inlight-emitting member 320 to increase the efficiency of the overalldevice 310. For example, these additional layers can serve to improvethe injection (electron or hole injection enhancement layers) ortransport (electron or hole transport layers) of charges into theorganic EL layer. The thickness of each of these layers is kept to below500 nm, preferably below 100 nm. Materials for these additional layersare typically low-to-intermediate molecular weight (less than about2000) organic molecules. They may be applied during the manufacture ofthe device 310 by conventional methods such as spray coating, dipcoating, or physical or chemical vapor deposition. In one embodiment ofthe present invention, as shown in FIG. 10, a hole injection enhancementlayer 336 is formed between the anode layer 338 and the organic EL layer330 to provide a higher injected current at a given forward bias and/ora higher maximum current before the failure of the device. Thus, thehole injection enhancement layer facilitates the injection of holes fromthe anode. Suitable materials for the hole injection enhancement layerare arylene-based compounds disclosed in U.S. Pat. No. 5,998,803; suchas 3,4,9,10-perylenetetra-carboxylic dianhydride orbis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole).

In another embodiment of the present invention, as shown in FIG. 11,light-emitting member 320 further includes a hole transport layer 334which is disposed between the hole injection enhancement layer 336 andthe organic EL layer 330. The hole transport layer 334 has the functionsof transporting holes and blocking the transportation of electrons sothat holes and electrons are optimally combined in the organic EL layer330. Materials suitable for the hole transport layer are triaryldiamine,tetraphenyldiamine, aromatic tertiary amines, hydrazone derivatives,carbazole derivatives, triazole derivatives, imidazole derivatives,oxadiazole derivatives having an amino group, and polythiophenes asdisclosed in U.S. Pat. No. 6,023,371, which is incorporated herein byreference.

In still another embodiment of the present invention, as shownschematically in FIG. 12, light-emitting member 320 includes anadditional layer 324 which is disposed between the cathode layer 322 andthe organic EL layer 330. Layer 324 has the combined function ofinjecting and transporting electrons to the organic EL layer 330.Materials suitable for the electron injecting and transporting layer aremetal organic complexes such as tris(8-quinolinolato)aluminum,oxadiazole derivatives, perylene derivatives, pyridine derivatives,pyrimidine derivatives, quinoline derivatives; quinoxaline derivatives,diphenylquinone derivatives, and nitro-substituted fluorene derivatives,as disclosed in U.S. Pat. No. 6,023,371, which is incorporated herein byreference.

A reflective metal layer 360 may be disposed on organic EL member 320 toreflect any radiation emitted away from the substantially transparentsubstrate 340 and direct such radiation toward the substrate 340 suchthat the total amount of radiation emitted in this direction isincreased. Reflective metal layer 360 also serves an additional functionof preventing diffusion of reactive environmental elements, such asoxygen and water vapor, into the organic EL element 320. Such adiffusion otherwise can degrade the long-term performance of the OELD.Suitable metals for the reflective layer 360 are silver, aluminum, andalloys thereof. It may be advantageous to provide a thickness that issufficient to substantially prevent the diffusion of oxygen and watervapor, as long as the thickness does not substantially reduce theflexibility of the entire device. In one embodiment of the presentinvention, one or more additional layers of at least a differentmaterial, such as a different metal or metal compound, may be formed onthe reflective layer to further reduce the rate of diffusion of oxygenand water vapor into the organic EL member. In this case, the materialfor such additional layer or layers need not be a reflective material.Compounds, such as metal oxides, nitrides, carbides, oxynitrides, oroxycarbides, may be useful for this purpose.

In another embodiment of the present invention, as shown in FIG. 13, abonding layer 358 of a substantially transparent organic polymericmaterial may be disposed on the organic EL member 320 before thereflective metal layer 360 is deposited thereon. Examples of materialssuitable for forming the organic polymeric layer are polyacrylates suchas polymers or copolymers of acrylic acid, methacrylic acid, esters ofthese acids, or acylonitrile; poly(vinyl fluoride); poly(vinylidenechloride); poly(vinyl alcohol); copolymer of vinyl alcohol and glyoxal(also known as ethanedial or oxaaldehyde); polyethyleneterephthalate,parylene (thermoplastic polymer based on p-xylene), and polymers derivedfrom cycloolefins and their derivatives (such as poly (arylcyclobutene)disclosed in U.S. Pat. Nos. 4,540,763 and 5,185,391 which areincorporated herein by reference). Preferably, the bonding layermaterial is an electrically insulating and substantially transparentpolymeric material. A suitable material is polyacrylates.

In another embodiment of the present invention, as shown in FIG. 14, asecond polymeric substrate 370 having a graded-composition barriercoating 372 is disposed on organic EL member 320 opposite to substrate340 to form a complete seal around organic EL member 320.Graded-composition barrier coating 372 may be disposed on either side ofsubstrate 370. It may be preferred to dispose graded-composition barriercoating 372 adjacent to organic EL member 320. Second polymericsubstrate 370 having graded-composition barrier coating 372 may also bedisposed on reflective metal layer 360 to provide even more protectionto organic EL member 320. Alternatively, graded-composition barrier 372may be deposited directly on organic EL member 320 instead of beingdisposed on a second polymeric substrate (such as 370). In this case,the second substrate (such as 370) may be eliminated.

Alternatively, second substrate 370 having graded-composition barriercoating 372 can be disposed between organic EL member 320 and reflectorlayer 360. This configuration may be desirable when it can offer somemanufacturing or cost advantage, especially when the transparency ofcoated substrate 370 is also substantial.

In another embodiment of the present invention, the light-emittingdevice 310 further comprises a light-scattering material disposed in thepath of light emitted from the light-emitting device 310 to provide moreuniform light therefrom. For example, FIG. 15 illustrates an embodimentcomprising a layer 390 of scattering material disposed on the substrate340. The light-scattering material is provided by choosing particlesthat range in size from about 10 nm to about 100 micrometers. Apreferred embodiment includes particles about 4 micrometers in size. Forexample, for a device emitting white light, the particle size ispreferably on the order of 50-65 nm. Particles of the light-scatteringmaterial may be advantageously dispersed in a substantially transparentpolymeric film-forming material such as those disclosed above, and themixture is formed into a film which may be disposed on the substrate340. Suitable light-scattering materials are solids having refractiveindex higher than that of the film forming material. Since typical filmforming materials have refractive indices between about 1.3 to about1.6, the particulate scattering material should have a refractive indexhigher than about 1.6 and should be optically transparent over thetarget wavelength range. In addition, it is preferable that the lightscattering material be non-toxic and substantially resistant todegradation upon exposure to normal ambient environments. For a devicedesigned to provide visible illumination (wavelength in the range ofabout 400-700 nm), examples of suitable light-scattering materials arerutile (TiO₂), hafnia (HfO₂), zirconia (ZrO₂), zircon (ZrO₂.SiO₂),gadolinium gallium garnet (Gd₃Ga₅O₁₂), barium sulfate, yttria (Y₂O₃),yttrium aluminum garnet (“YAG”, Y₃Al₅O₁₂) calcite (CaCO₃), sapphire(Al₂O₃), diamond, magnesium oxide, germanium oxide. It is necessary toprepare these compounds with a high degree of optical purity; i.e.impurities that absorb light in the wavelength range of interest must berigorously minimized. It is not necessary that the compound bestoichiometrically pure, phase pure, and may contain appropriate atomicsubstitutions; e.g., Gd may be substituted for up to 60% of the yttriumin YAG. Particles composed of high-refractive index glasses, such as maybe obtained from Schott Glass Technologies or Corning, Inc. may also beused, provided that they are impervious to darkening from exposure tolight emitted by the OELD and its phosphors. Scattering of light mayalso be achieved with a plastic or glass film having a roughened ortextured surface (a “diffuser film”, the roughened features of which aretypically on the order of a fraction of the wavelength of the scatteredlight. In one embodiment of the present invention, one surface of thesubstrate can be textured or roughened to promote light scattering.

According another aspect of the present invention, the light-scatteringparticles in layer 390 can comprise a photoluminescent (“PL”) material(or also herein called a “phosphor”), which is capable of absorbing aportion of the EM radiation emitted by the organic EL member having afirst wavelength range and emitting EM radiation having a secondwavelength range. Thus, inclusion of such a PL material can provide atuning of color of light emitted from the OELD. The particle size andthe interaction between the surface of the particle and the polymericmedium determine how well particles are dispersed in polymeric materialsto form the film or layer 390. Many micrometer-sized particles of oxidematerials, such as zirconia, yttrium and rare-earth garnets, andhalophosphates, disperse well in standard silicone polymers, such aspoly(dimethylsiloxanes) by simple stirring. If necessary, otherdispersant materials (such as a surfactant or a polymeric material likepoly(vinyl alcohol)) may be added such as are used to suspend manystandard phosphors in solution. The phosphor particles may be preparedfrom larger pieces of phosphor material by any grinding or pulverizationmethod, such as ball milling using zirconia-toughened balls or jetmilling. They also may be prepared by crystal growth from solution, andtheir size may be controlled by terminating the crystal growth at anappropriate time. The preferred phosphor materials efficiently absorb EMradiation emitted by the organic EL material and re-emit light inanother spectral region. Such a combination of the organic EL materialand the phosphor allows for a flexibility in tuning the color of lightemitted by the light-emitting device 310. A particular phosphor materialor a mixture of phosphors may be chosen to emit a desired color or arange of color to complement the color emitted by the organic ELmaterial and that emitted by the organic PL materials. An exemplaryphosphor is the cerium-doped yttrium aluminum oxide Y₃Al₅O₁₂ garnet(“YAG:Ce”). Other suitable phosphors are based on YAG doped with morethan one type of rare earth ions, such as (Y_(1-x-y)Gd_(x)Ce_(y))₃Al₅O₁₂(“YAG:Gd,Ce”), (Y_(1-x)Ce_(x))₃ (Al_(1-y)Ga_(y))O₁₂ (“YAG:Ga,Ce”),(Y_(1-x-y)Gd_(x)Ce_(y))(Al_(5-z)Ga_(z))O₁₂ (“YAG:Gd,Ga,Ce”), and(Gd_(1-x)Ce_(x))Sc₂Al₃O₁₂ (“GSAG”) where 0≦x≦1, 0≦y≦1, 0≦z≦5 and x+y≦1.For example, the YAG:Gd,Ce phosphor shows an absorption of light in thewavelength range from about 390 nm to about 530 nm (i.e., the blue-greenspectral region) and an emission of light in the wavelength range fromabout 490 nm to about 700 nm (i.e., the green-to-red spectral region).Related phosphors include Lu₃Al₅O₁₂ and Tb₂Al₅O₁₂, both doped withcerium. In addition, these cerium-doped garnet phosphors may also beadditionally doped with small amounts of Pr (such as about 0.1-2 molepercent) to produce an additional enhancement of red emission. Thefollowing are examples of phosphors that are efficiently excited by EMradiation emitted in the wavelength region of 300 nm to about 500 nm bypolysilanes and their derivatives.

-   -   Green-emitting phosphors: Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺, Mn²⁺;        GdBO₃:Ce³⁺, Tb³⁺; CeMgAl₁₁O₁₉:Tb³⁺; Y₂SiO₅:Ce³⁺, Tb³⁺; and        BaMg₂Al₁₆O²⁻:Eu²⁺, Mn²⁺.    -   Red-emitting phosphors: Y₂O₃:Bi³⁺, Eu³⁺; Sr₂P₂O₇:Eu²⁺, Mn²⁺;        SrMgP₂O₇:Eu²⁺, Mn²⁺; (Y,Gd)(V,B)O₄:Eu³⁺; and 3.5        MgO.0.5MgF₂.GeO₂:Mn⁴⁺ (magnesium fluorogermanate).    -   Blue-emitting phosphors: BaMg₂Al₁₆O₂₇:Eu²⁺; Sr₅(PO₄)₁₀Cl₂:Eu²⁺;        and (Ba,Ca,Sr)₅(PO₄)₁₀(Cl,F)₂:Eu²⁺, (Ca,Ba,Sr)(Al,Ga)₂S₄:Eu²⁺.    -   Yellow-emitting phosphors: (Ba,Ca,Sr)₅(PO₄)₁₀(Cl,F)₂:Eu²⁺, Mn²⁺.

Still other ions may be incorporated into the phosphor to transferenergy from the light emitted from the organic material to otheractivator ions in the phosphor host lattice as a way to increase theenergy utilization. For example, when Sb³⁺ and Mn²⁺ ions exist in thesame phosphor lattice, Sb³⁺ efficiently absorbs light in the bluephosphors not absorbed very efficiently by Mn²⁺, and transfers theenergy to Mn²⁺ ion. Thus, a larger total amount of light emitted by theorganic EL material is absorbed by both ions, resulting in higherquantum efficiency of the total device.

The photoluminescent material may also be an organic dye that can absorbradiation emitted by the organic EL material and emit electromagneticradiation in the visible spectrum.

The phosphor particles are dispersed in a film-forming polymericmaterial, such as polyacrylates, substantially transparent silicone orepoxy. A phosphor composition of less than about 30, preferably lessthan about 10, percent by volume of the mixture of polymeric materialand phosphor is used. A solvent may be added into the mixture to adjustthe viscosity of the film-forming material to a desired level. Themixture of the film-forming material and phosphor particles is formedinto a layer by spray coating, dip coating, printing, or casting on asubstrate. Thereafter, the film is removed from the substrate anddisposed on the light-emitting member 320. The thickness of film orlayer 390 is preferably less than 1 mm, more preferably less than 500μm. Preferably, the film-forming polymeric materials have refractiveindices close to those of the substrate 340 and the organic EL material;i.e., in the range from about 1.4 to about 1.6.

According to one aspect of the present invention, particles of ascattering material and a phosphor are dispersed in the same film orlayer 390. In another embodiment, scattering film 390 may be a diffuserfilm, which is a plastic film having a roughened surface.

A method of making an OELD of the present invention is now described. Acleaned flexible substrate, such as a plastic, is first provided. Then,a graded-composition barrier coating is formed on at least a surface ofthe flexible substrate by a one of many deposition techniques disclosedabove.

A first electrically conducting material is deposited on thegraded-composition barrier coating to form a first electrode of theorganic EL member 320. Alternatively, the first electrode may bedeposited on the surface of the substrate 340 that has not been coatedwith graded-composition barrier coating. The first electrode may be ananode or a cathode, and one or more appropriate materials are chosenamong those disclosed earlier for the electrodes. Preferably, the firstelectrode is an anode comprising a transparent metal oxide, such as ITO.The first electrode material preferably sputter-deposited on thesubstrate. Furthermore, the first electrode may be patterned to adesired configuration by, for example, etching. At least one organic ELmaterial is deposited on the first electrode by physical or chemicalvapor deposition, spin coating, dip coating, spraying, printing, orcasting, followed by polymerization, if necessary, or curing of thematerial. The organic EL material may be diluted in a solvent to adjustits viscosity or mixed with another polymeric material that serves as afilm-forming vehicle. A second electrically conducting material isdeposited on the at least one organic EL material to form a secondelectrode. Preferably, the second electrode is a cathode. The secondelectrode may be deposited on the entire area of the organic EL materialor patterned into a desired shape or configuration. The thickness of thesecond electrode is kept to a minimum, such as less than or equal toabout 200 nm. The electrodes and the organic EL material comprise theorganic EL member 320.

A reflective metal is optionally deposited on the surface of the organicEL member 320 opposite to substrate 340. The reflective metal may bedeposited by, for example, sputtering or physical vapor deposition. Inone embodiment of the present invention, a bonding layer of asubstantially transparent material is deposited on the organic EL member320 before the layer of reflective metal is deposited thereon.Preferably, the bonding layer comprises an electrically insulating andsubstantially transparent polymeric material. The bonding layer may bedeposited by one of the methods disclosed above for deposition of anorganic layer. The reflective metal layer is formed so as to completelysurround the organic EL member 320. Preferably, the reflective metallayer together with the graded-composition barrier coating forms ahermetic seal around the organic EL member 20. Furthermore, one or moreadditional layers of other inorganic materials may be deposited on thereflective metal layer.

A mixture of particles of a scattering or PL material and a transparentpolymeric material is deposited on the surface of the substrate 340opposite the organic EL member. Alternatively the mixture may be castinto a tape by a tape casting method, such as the doctor blade method.The tape is then cured and attached to the substrate 340.

In another embodiment, subsets of layers necessary or desired for theoperation of an OELD of the present invention are formed in separateassemblies, and the assemblies are laminated or attached together toproduce a working device. For example, a first substrate having a firstgraded-composition barrier coating, an assembly of an organic EL member,and a second substrate having a second graded-composition barriercoating are laminated together to provide a light source having improvedresistance to attack by chemical species in the environment.

In still another aspect of the present invention, large-area flexibledisplays or lighting systems incorporate OELDs of the present invention.

While specific preferred embodiments of the present invention have beendisclosed in the foregoing, it will be appreciated by those skilled inthe art that many modifications, substitutions, or variations may bemade thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

The invention claimed is:
 1. A method for making a composite article,said method comprising: providing a substrate having at least asubstrate surface; depositing a coating material comprising an inorganicand an organic material, said coating material having a composition onsaid substrate surface; and changing said composition of said coatingmaterial substantially continuously while said coating is being formedsuch that said composition varies substantially continuously across athickness of said coating; wherein the composition of said coatingmaterial varies across a thickness thereof such that a graph of thepercent composition of one of the components of the coating versus thedepth of the coating has plural peaks.
 2. The method according to claim1, wherein said depositing is selected from the group consisting ofplasma-enhanced chemical-vapor deposition, radio-frequencyplasma-enhanced chemical-vapor deposition, expanding thermal-plasmachemical-vapor deposition, sputtering, reactive sputtering,electron-cyclotron-resonance plasma-enhanced chemical-vapor deposition,inductively-coupled plasma-enhanced chemical-vapor deposition, andcombinations thereof.
 3. The method according to claim 1, wherein saidsubstrate comprises a polymeric material selected from the groupconsisting of polyethyleneterephthalate, polyacrylates, polycarbonate,silicone, epoxy resins, silicone-functionalized epoxy resins, polyester,polyimide, polyetherimide, polyethersulfone, polyethylenenapthalene,polynorbonene, poly(cyclic olefins).
 4. The method according to claim 1,wherein said coating material further comprises material selected fromthe group consisting of organic, inorganic, ceramic materials, andcombinations thereof.
 5. The method according to claim 4, wherein saidinorganic and ceramic materials are selected from the group consistingof oxide, nitride, carbide, boride, and combinations thereof of elementsof Groups IIA, IIIA, IVA, VA, VIA, VIIA, IB, and IIB, metals of GroupsIIIB, IVB, and VB, and rare-earth metals.
 6. The method according toclaim 1 further comprising effecting a penetration of at least a portionof said coating material into said substrate to produce a diffuse regionbetween said substrate and said coating.
 7. The method according toclaim 6, wherein said diffuse region is produced by an energetic ionbombardment of a surface of said substrate to sputter a portion of amaterial of said substrate, and depositing a mixed material comprisingsputtered substrate material and another material.
 8. The methodaccording to claim 1, wherein said changing said composition of saidcoating is carried out by changing a composition of reactive speciesgenerated by a plasma that is directed at said substrate.
 9. The methodaccording to claim 1 wherein said plural peaks are substantially thesame height.
 10. The method according to claim 1 wherein the pluralityof peaks is at least three.
 11. The method according to claim 10 whereinthe height of the peaks changes as a function of depth.